The invention relates generally to a composition capable of rigidity tuning. More specifically, the invention relates to a composite comprising a conductive elastomer that reversibly changes rigidity through the use of an electrical current.
Materials capable of rapid and reversible changes in mechanical rigidity have applications ranging from nature to engineered systems. Natural organisms typically perform rigidity tuning with internal structures such as striated muscle or hydroelastic skeletons. These natural composites and structures are lightweight and require only ˜0.1-1 seconds to change their tensile rigidity by 1-2 orders of magnitude. In engineered systems, rigidity tuning is often accomplish with external hardware and finds applications from damping/vibration control, active orthoses, robotic grippers, artificial muscles, and wearable technology.
Rigidity tuning allows a material to remain elastic at times, while providing support at other times. In an elastic material, the maximum mechanical stress that the material can support is approximately proportional to its elastic (Young's) modulus (E). Therefore, an elastomer with a modulus of 1 MPa can only exhibit approximately 1/1000th the load-bearing capability of a rigid plastic (˜1 GPa). Nature deals with the challenge of providing elasticity and support through materials like muscle and catch connective tissue, natural materials that actively change their elastic rigidity by tuning the internal sliding friction between inextensible myofilaments and collagen fibers, respectively.
For engineered systems, the external hardware used to change rigidity can be appropriate for relatively large machinery, but these mechanisms cannot be easily scaled for clothing-embedded technologies, small-scale robotics, and other applications that depend on miniaturization and autonomous operation. Examples of such hardware include pumps and valves for gel hydration or pneumatic particle jamming, electromagnets for activating magnetorheological fluids and elastomers, and high voltage activation for electroactive polymers.
Alternatively, efforts have been made to utilize thermal activation of non-conductive shape memory polymers, thermoplastics, coiled fibers, wax-soaked thermoplastics, and low-melting-point alloys. Many of these materials use an external heating element or employ direct Joule heating, such as microfluidic channels of liquid-phase metals, including gallium-indium (GaIn) alloy to reach a glass transition or melting temperature. While promising in certain applications, the separate Joule heating element must remain electrically functional during elastic deformation and mechanical load. In addition, the use of liquid GaIn introduces sealing issues and requires separation from the thermally-responsive material, adding layers and sources of heat dissipation to the composite.
In the techniques mentioned above, external hardware or the use of low-melting point alloys create complications for many applications. It would therefore be advantageous to develop a rigidity tunable composite that does not rely on liquid metals or bulky hardware to accomplish changes in rigidity.